This invention relates to a method of controlling alumina feed to an aluminum electrolytic cell, and more particularly a method of controlling alumina feed when alumina is fed continuously to the aluminium electrolytic bath. Especially, the invention relates to a method of controlling alumina feed capable of performing electrolytic operation at high efficiencies while preventing rapid change in the concentration of alumina in the electrolytic bath.
To prepare a aluminum at a commercial scale, alumina is electrolyzed in an electrolytic cell utilizing an electrolytic bath consisting mainly of cryolite, whereby alumina is reduced to aluminum metal.
During the normal operation of an aluminum electrolytic cell, the concentration of alumina in the electrolytic bath is generally maintained in a range of from 2 to 8% by weight. When the alumina concentration in the electrolytic bath exceeds its saturation value, the alumina in the bath deposits on the upper surface of the cathode electrode thus forming so-called sludge. The current efficiency of the electrolytic cell in which sludge has been formed decreases greatly and it takes a long time before normal state is resumed.
On the other hand, when the alumina concentration in the electrolytic bath decreases below a definite limit, the electrolytic bath per se undergoes electrolysis thus forming a gas film on the lower surface of the anode electrode immersed in the bath. When gas film builds up the interelectrode resistance increases, thus resulting in a so-called anode effect in which the electrolysis cell voltage increases rapidly. The cell voltage of a cell in which the anode effect occurred increases to about 30 to 40 volts which should be compared with the cell voltage of about 4 to 5 volts of a cell under normal operating condition, which means a large power loss. Such anode effect, however, can be eliminated by feeding alumina to the electrolytic cell, thus increasing the alumina concentration in the bath.
The anode effect does not present serious trouble because the normal condition can be resumed in a much shorter time than when sludge is formed. Accordingly, the electrolytic cell is generally operated over a range in which the alumina concentration is relatively low so as to prevent as far as possible the deposition of alumina.
The temperature of the electrolytic bath, however, is considerably high, of the order of 940.degree. to 1000.degree. C. and moreover since the alumina concentration in the bath varies with time, so it is difficult to control the alumina concentration by directly measuring the alumina concentration in the electrolytic bath.
Generally, the feed of alumina into the electrolytic bath is done either by breaking, at a definite interval, a suitable quantity of a crust of frozen electrolyte which is covering the bath, thereby causing the alumina to be forced into the bath together with broken crust to dissolve them in the bath or by continuously feeding a suitable quantity of alumina directly into the bath by an aluminum feeding device, thereby dissolving alumina.
According to these methods, however, as the alumina is fed independently of the alumina concentration of the electrolytic bath, there is a tendency of forming sludge due to excessive feed of alumina or of frequent occurrence of the anode effect due to deficient feed of alumina. Where the crust is broken at a definite interval for forcing and dissolving alumina into the bath, the alumina concentration of the bath varies greatly in a short time thus decreasing the current efficiency.
As a result of investigation made on the variation in the alumina concentration in an electrolytic bath during the electrolysis operation, we have found that, so long as the condition of electrolysis is substantially constant as is the case during normal operation, the alumina concentration in the electrolytic bath is substantially constant when anode effect occurs and that so long as the method of anticipating the anode effect is definite, the alumina concentration in the bath is also substantially constant at a time when the anode effect is anticipated. For example, according to a test result made on a prebake type aluminum electrolytic cell operating at an average anode current density of 0.72 A/cm.sup.2, the alumina concentration in the electrolytic bath when anode effect occurs is always equal to 1.0 .+-. 0.2% whereas the alumina concentration is about 1.7 .+-. 0.3% when the occurrence of the anode effect is anticipated.
Based on these facts, according to this invention, alumina is fed continuously by taking as a reference the alumina concentration in the electrolytic bath when the occurrence of the anode effect is detected or anticipated and by taking into consideration the weighing errors of an alumina feeding device, thereby preventing rapid variation in the alumina concentration in the electrolytic cell as well as frequent occurrence of the anode effect and the formation of sludge.